In typical 35 mm film cartridges, the filmstrip is withdrawn by its "film leader" from the cartridge and advanced past a rectangular exposure window or space gate and wound up on a take up reel until all potential image frames of the filmstrip are exposed. More recently there has been introduced, "thrust" cartridges, wherein the filmstrip may be advanced out of the light trapping slit or mouth of the cartridge shell by rotating the cartridge spool in an unwinding direction as is described in commonly assigned U.S. Pat. No. 4,933,693, incorporated herein by reference in its entirety. In use, the thrust cartridge may be loaded in a camera without an exposed leader protruding from the shell slit and advanced, after the camera film loading door is closed, by motorized drive mechanism until the first image frame is positioned for exposure. The thrust cartridge avoids any unnecessary waste of film leader and initial frames on loading into the camera. Moreover, it is contemplated that developed negative filmstrips may be returned to the customer wound back into the cartridge for safekeeping. In any case, the filmstrip is automatically or mechanically rewound into the film cartridge of either type upon exposure of all image frames, and the cartridge is rewound for processing and printing or mounting, in the case of positive film.
Before the filmstrip is completely exposed, it may reside tightly wound in the cartridge or partially unwound within the camera for a period of time and under varying conditions of temperature and humidity such that the filmstrip takes a "core set affecting the degree to which the film curls back upon itself when unrestrained". When filmstrip is originally manufactured and cut to length it is generally fairly limp and relatively easy to maintain straight (because it is wound on a relatively large diameter core maintained at a 50-60% relative humidity). After winding onto the spool of the film cartridge, the innermost layers of the filmstrip around the spool take on a higher degree of curl (given the relatively smaller diameter) then the outermost layers and the degree of curl becomes more pronounced with the passage of time and exposure to elevated temperature and/or low humidity. Moreover, with use of the thrust cartridges to store developed film, core set film curl may become more likely and more pronounced with the passage of time and be more apparent when prints are re-ordered from the aged filmstrip. This is particularly true if the spool diameter is small and the film base is thin, relative to the thickness of the emulsion layers, in order to miniaturize dimensions of the cartridge. Additionally, the material from which the film base is made can significantly affect the film's ability to take a core set. In particular, film having a PET (polyethylene terephthalate) base has been found to be very susceptible to core set.
Thus, the degree of core set film curl and its frequency of occurrence varies considerably due to its type, age, and environment to which it has been exposed in use and storage. This inconsistent core set film curl may make it difficult for photofinishing equipment to handle the film during splicing, processing and printing or mounting. In certain photofinishing operations, individual filmstrips are withdrawn or advanced from the film cartridge and passed through developing and fixing baths during processing, wherein the transport mechanism does not restrain the free end of the filmstrip. A number of film based materials absorb fluids in the bath become more limp when they are first withdrawn from the cartridge. Other film based materials are more hydrophobic, and the excess film curl, particularly the more hydrophobic filmstrips, may cause film to roll up or be folded back upon itself and be damaged as it sticks together or may jam film transporting mechanisms in the processing, splicing, printing or mounting equipment. The film handling the characteristics of the transporting mechanism may vary widely and are typically preset or configured to accommodate an "average" film curl and are not readily adjustable. There is a high probability that the film will be damaged if the device does not properly account for the curl present in the film. Thus, the equipment must be monitored and halted if a malfunction occurs.
One solution to the problem would be to subject all filmstrips to decurling operations upon withdrawal from the filmstrip cartridge and prior to processing, printing or mounting. It is known that sheet material may be decurled by passing the sheet over a sharp radius. This process is highly undesirable for use with film due to the possibility of severely damaging the film and/or image thereon. In order to make decurling of all filmstrips commercially feasible, the device must be reliable, low cost and easy to use without inflicting any adverse damage to the filmstrip. Japanese Patent Application No. P4099448 discloses heating of the filmstrip about a roller in a direction opposite the curl and heating the filmstrip above its glass transition temperature. Heating of the filmstrip to such temperatures present a high risk of causing sensitometric damage to the film. Additionally, the cooling provided is not very controlled.
Applicants have invented a decurling device and method of transporting the filmstrip which does so in a compact and efficient manner which is reliable, relatively low cost and does not require any significant controls.